Known from prior art are various extrusion methods in which plastic starting material is melted, mixed with optional additives and colorants and extruded through a die of a certain profile in order to form a product. When emerging from the die, the product is in, or close to, its final shape. From the die, the product can be lead to a calibration unit, where it is provided accurate and final dimensions, and further to a water reservoir functioning as a cooler. The product is sawed into the desired size on the production line. The starting material is most often fed into the extruder in granulate or powder form.
Also known from the prior art are different wood-plastic composite materials and methods for making them. Wood-plastic composite materials are made mainly from sawdust and plastic which are usually fed into the extrusion as granulates. The wood-based starting material as well as the plastic must therefore be first extruded in separate processes into granulates. However, it is possible to feed the starting material components into the extruder as such and make them into products. This requires the use of different kinds of force feed devices to ensure a steady feed despite the heterogeneity and low specific density of the starting material. The method is called direct feed.
Wood-plastic composites made by the extrusion method are manufactured by adding a wood-based constituent and different processing auxiliaries as fillers to the matrix plastic. The wood-based constituents used are typically sawdust but may also be wood chips or chemical pulp fibre. The share of the added wood-based constituent in the plastic composite is from 5 to 90%, varying greatly according to the application. For example, in a product intended for outdoor use, the share of the wood-based constituent is less than 50%. The plastic used is most typically polyolefin, such as PP, LDPE or HDPE, but it may alternatively be PVC, PS or some other thermoplastic polymer, or a mixture of different plastics. The plastic used may be virgin or recycled plastic.
When it is desired that the wood-based constituent added to the matrix reinforces the profile, the fibre length and shape of the added constituent must be taken into account. The sawdust used in wood-plastic composites is so shaped that it does not actually reinforce the product but instead functions merely as filler in the plastic matrix. The size of the sawdust typically used in wood-plastic composites is in the range of 40 to 60 mesh (from 250 to 425 μm), such that when it is used, some of the original length of the wood fibre is lost already at the production site of the starting material, such as a sawmill. Moreover, the starting materials of the wood-plastic composites made from sawdust are pre-processed by granulation before the actual processing. Such mechanical stress shortens the wood fibres even more, and the additional processing also impairs the properties of the matrix plastic. Additional processing also increases production costs.
Sawdust or wood chips normally require drying before the actual processing, because high moisture content in the filler weakens the quality of the manufactured product. The water vapour which is gasified in the processing produces gas bubbles to the profile, which affect the look of the product as well as its mechanical properties. The drying performed as an additional process is an unwanted step due to additional costs, but also to the risk of explosion.
The traditional wood-plastic composites have relatively high water absorbency and swelling compared to pure plastics. This is caused by hydrophilicity of the wood fibres and the relatively large surface area of the sawdust particles. It is commonly known that in a composite intended for outdoor use, the share of the wood-based constituent may not exceed 45%, being preferably less than 40%. The price of the composite is determined by the share and price of the required matrix plastic, such that demanding moisture conditions at the application site raise the price of the product considerably.
Known from prior art is also manufacture of wood composites from wood waste arranged in the form of pellets, and from recycled plastic. Known from publication U.S. Pat. No. 4,874,095 is also manufacture of wood composite materials from paper and thermoplastic material, and from publications US 20020034629 and U.S. Pat. No. 3,718,536 manufacture of wood composite material from wood- and plastic-based materials.